Method for converting direct voltage into three phase alternating voltage

ABSTRACT

A method of converting a direct voltage generated by a decentralized power supply system into three-phase alternating voltage by means of a plurality of single-phase inverters (WR 1 -WR 3 ), said alternating voltage being provided for supplying an electric mains, is intended to avoid inadmissible load unbalances using single-phase inverters. This is achieved in that, upon failure of one inverter (WR 1 -WR 3 ), an asymmetrical power supply distribution is reduced by limiting the output of the other inverters. The method makes it possible to simplify three-phase voltage monitoring.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims Priority from German Application No. DE 10 2006003 904.1-32 filed on 27 Jan. 2006

FIELD OF THE INVENTION

The invention relates to a method having the features recited in thepreamble of claim 1.

DESCRIPTION OF THE PRIOR ART

Mains connected systems for decentralized energy supply, e.g.,photovoltaic systems with solar modules and inverters or systems withfuel cells and inverters are known. For low output, a single-phaseinverter is usually used for single-phase power supply; for high output,the power supply is three-phased. However, the use of a three-phaseinverter is expensive, since it has less efficiency and is produced insmaller quantities.

This is the reason why, for high output, one usually uses threesingle-phase inverters. However, it suffices a failure of but one of theinverters, as a result of a short for example, to often causeasymmetrical power supply exceeding the admissible limit or a loadunbalance to occur as a result of the independent supply.

BRIEF SUMMARY OF THE INVENTION

It is the object of the invention to indicate a method that avoidsinadmissible load unbalances using single-phase inverters.

This object is solved with the characterizing features of claim 1 inconnection with the features recited in the preamble thereof as well aswith the characterizing features of claim 9 in connection with thefeatures of the preamble thereof.

The invention relies on the idea consisting in limiting asymmetricalpower supply to an admissible limit and not to switch off the entiresystem so that the intact inverters are capable of proceeding withfeeding in the associated phases. In not shutting down the entiresystem, the quality of current supply or the current availabilityincreases. This means that, in accordance with the invention, the powerof the other two inverters is initiated to an imposed value when onlyone of the inverter fails, for what reason so ever, e.g., as a result ofa short circuit. This value may be greater than or equal to zero.

The invention allows for improving the quality of the electric mainswhile tolerating an admissible load unbalance, with single-phaseinverters permitting to achieve high efficiency, modularity and low-costmanufacturing.

In an advantageous developed implementation of the method of theinvention, there is provided that the power of every inverter be limiteddurably to 2-20 kVA, more specifically to about 4.6 kVA. There ispreferably provided that the performance of every inverter be limitedtemporarily, more specifically for about 10 minutes, to about 5 kVA.This measure is particularly suited for mains connected systems for thedecentralized energy supply with inverters having a higher rated output,more specifically having an output greater than 4.6 kVA.

It is advantageous if the failure of the inverter is detected by ameasurement signal of an additional circuit of the inverter, a powerlimiting communication signal resulting from the measurement signalbeing provided. The measurement signal simply detects the failure of oneinverter, with such an additional circuit being housed in an additionalmounting plate on each of the inverters. This allows for utilizingcommercially available single-phase inverters that only need littleconversion to implement the method of the invention.

In order to limit the power of the inverters, there is practicallyprovided that the communication signal is communicated to the processorsof the inverters in order to limit the power of the inverters bycontrolling the processors. The processor or microprocessor in each ofthe intact phases receives a command to limit the power and the invertermay produce the power needed through a corresponding PWM control forexample.

Another advantageous measure is characterized in that the measurementsignal is produced by a current from a constant current source, saidcurrent of said constant current source being in the milliampere range.Through the low measurement current that may preferably be produced ineach additional mounting plate, it is possible to put into practice thedetection of the failure, using little additional power or rather withhigh efficiency. It is particularly advantageous if, upon failure of theinverter, the fault is communicated through one or a plurality ofexternal cables between the inverters. As a result, the independentsingle-phase inverters may practically communicate together so that thestatus of an inverter is recognized. In case of failure of one or aplurality of inverters, the inventive power limitation of the otherinverters is realized.

Other advantageous developed implementations of the invention willbecome apparent from the dependent claims.

The invention will be better understood upon reading the more detaileddescription of the Figures, which describes other advantages thereof.

In said Figures:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of a circuit arrangement of theinvention for limiting a load unbalance,

FIG. 2 shows a circuit with two groups of single-phase inverters with amains contactor and a three-phase voltage monitoring module,

FIG. 3 shows a circuit with two groups of single-phase inverters with amains contactor without three-phase voltage monitoring module and

FIG. 4 shows a circuit with two groups of single-phase inverters, themains being disconnected directly by the inverters.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example with three single-phase inverters WR1, WR2, WR3,more specifically with photovoltaic inverters, which supply three-phasealternating voltage to a low-voltage main system. As will be shownlater, the inverters convert a decentralized direct voltage, produced bysolar modules in particular, into a standardized low voltage of230V/400V/50 Hz for example.

Every inverter WR1-WR3 is provided with a circuit that forms what isreferred to as an intertripping or a failure measuring circuit 1. Thefailure measuring circuit 1 is an additional electronic circuit thatconnects the three single-phase inverters WR1-WR3 through an externalconnection 2. Upon failure of one inverter, this failure is communicatedbetween the inverters WR1-WR3 through one or a plurality of externalcables 5.

The task of the circuit shown is to detect the failure of an inverter sothat, upon asymmetrical power supply resulting from the failure of oneinverter, the power of the other inverters may be lowered in order toreduce load unbalance to an admissible limit. In permanent operation,the individual power of the inverters WR1-WR3 is to be limited to 4.6kVA for example.

Each measuring circuit 1 includes a direct current source 3, morespecifically a constant current source. Said source can be connectedadditionally through an electrically conductive bridge 4 so that onlyone of the current sources 3 delivers a measurement current, as can beseen in FIG. 1. Further, each measuring circuit includes a connectionfor a measurement and/or an output signal, which has not beenillustrated in greater detail herein. As a result, a failure signal oran OK-signal can be communicated to the other two operative inverters.

It is preferred that every inverter WR1-WR3 be provided with anadditional circuit, each inverter having the same additional circuit.The circuit has connections for the direct current source and for thecommunication lines. One of the current sources delivers the measurementcurrent for measuring the failure, said measurement current being a fewmA. In FIG. 1, it is the inverter WR1 that delivers the measurementcurrent. Every inverter however has the possibility of communicating afailure or an OK signal to the other two inverters through the outputsignal. A clearing signal is only generated if the inverters are readyfor supply in all of the three phases. Then, all the inverterscommunicate an OK signal. The communication may be by radio.

In a circuit arrangement that has not been illustrated in greater detailherein and that is provided in the additional circuit, the failure ofthe inverter is registered by a measurement signal. A communicationsignal resulting from said measurement signal regulates the powerlimitation to a value of 4.6 kVA for example. For the communicationsignal is communicated to the processors of the inverters in order tolimit the power of the inverters by means of processor control or of PWMcontrol for example.

FIG. 2 shows a circuit with two groups GR1, GR2 of single-phaseinverters 11-16 that are connected in parallel for increasing the power.The solution in groups is lower in cost since single-phase inverters aremanufactured in large quantities and have greater efficiency.

Each inverter 11-16 is supplied, on its input side, with direct voltagegenerated by solar modules 9. One inverter practically generatesalternating current of one phase L1, L2 or L3. Each inverter is moreoverprovided with an external communication line 2.

For safety reasons, the system shown usually includes a switching centerwith disconnecting function or clearing means 18 that is accessible tothe personnel of the mains operator.

Switching stations may be overground connecting points of a service tapto the supply mains such as a cable connection box, a cable distributioncabinet, a transformer station or a service box, as long as it isunrestrictedly accessible to the personnel of the mains operator.

The clearing means 18 is connected to a contactor 19 in such a mannerthat said contactor 19 is capable of causing the mains to disconnectthrough actuation of the clearing means 18. Moreover, the contactor 19is coupled to a three-phase voltage monitoring module 20 so that anadditional network limiter is provided. As shown in FIG. 2, the clearingmeans 18 is connected to the three-phase voltage monitoring module 20.This module 20 allows for disconnecting all the inverters from the mainswhen the voltage of one phase exceeds, or falls short of, imposed limitvalues. Network limitation is further improved as a result thereof.

Preferably, a plurality of groups of three single-phased inverters areconnected to the output side of a three-phase protection switchgear(contactor 19), said switchgear being adapted to be switched off throughthe manual, external clearing means 18 or through a voltage monitoringmodule 20.

In the solution shown in FIG. 3, one has made the economy of the module20 so that the installation expense is reduced. The contactor 19 howeveris still connected to the clearing means 18.

As shown in FIG. 4, the failure circuit may further be implemented insuch a manner that a signal occurs from outside, for example byactuation of the clearing means 18, said signal causing the inverters11-16 to become disconnected from the mains. This can obviate the needfor the contactor 19, an air break switch disconnector or anotherswitchgear.

Accordingly, the devices shown for monitoring the mains are providedwith a respective associated switching member or with a clearingstation. With single-phase power supply through one of the inverters11-16 to the mains supply system, such a device is utilized up to arated output 4.6 kVA, in a three-phased implementation, up to an outputof 30 kVA for PV inverters (PV=photovoltaic). It may be integrated intothe PV inverter or implemented as an independent protective device. Inaddition thereto, the device may include a voltage and frequencymonitoring device and may evaluate as an additional criterion locatedmains impedance leaps of a certain order of magnitude. Thanks to thesecriteria, namely the redundant implementation of the switching membersand the self-monitoring of the measurement system, the device is capableof meeting safety demands so that the required switching center withdisconnection function, which has to be always accessible to thepersonnel of the mains operator, and the separate voltage and frequencymonitoring device otherwise provided for preventing decoupling can besimplified.

For PV-systems with rated outputs >30 kVA, both an always accessibleclearing station and a three-phase voltage monitoring device are needed.On systems with inverters, this is preferably realized in the followingmanner:

The system may be disconnected by the contactor 19 in the main line,which connects the system to the mains connecting point. As shown,groups of inverters are thereby formed in larger systems. Each of thesegroups can be disconnected from the mains by its own contactor. Savingscan thus be made since a high-capacity contactor is more expensive thana plurality of contactors having a lower switching capacity.Motor-driven switches are also possible as switchgears for protectingthe mains.

A separate voltage monitoring relay (module 20 and contactor 19)measuring the voltages in the three phases of the mains connection mayact onto the protector as shown in FIG. 2. If one voltage leaves theadjusted range, the voltage monitoring relay opens the contactor 19.

In order to implement the always accessible clearing station a turn knobsnap switch, which also acts onto the mains disconnecting contactor 19,can be housed on the outside of a house, for example in a lockable box.

If an inverter for single-phase power supply has a three-phase mainsmonitoring device, the voltage monitoring module 20 can be obviated. Ifthree inverters for single-phase power supply are distributed over threemains phases and are connected together using an intertripping or afailure circuit 1 for the other two inverters to also switch off uponfailure of one inverter, this corresponds to a three-phase voltagemonitoring. In this case, the voltage monitoring relay according to FIG.2 can be obviated.

The possibility of disconnecting the inverters 11-16 from the mainsthrough a contact from the outside is, in terms of construction, easy tocombine with the intertripping illustrated in FIG. 2; therefore thecurrent source 3 of the intertripping (see I_(mess) 1 in FIG. 2) can bepractically switched on and off through an external switch. The costsincurred by this additional function are minimal. In this case, the maindisconnecting contactor 19 can be obviated.

In the Figures, there has been described a method of converting a directvoltage generated by a photovoltaic system into a three-phasealternating current by means of a plurality of single-phase inverters,the alternating current being provided for being supplied to an electricmains supply. In accordance with the invention, upon failure of oneinverter, an asymmetrical power distribution of the mains electricitysupply is reduced by limiting the output of the other inverters.

We claim:
 1. A method of converting direct voltage generated by a mainsconnected system for decentralized power supply into a three-phasealternating voltage by means of a plurality of single-phase inverters(WR1-WR3) connected to a different phase (L1, L2, L3), said alternatingvoltage being provided for supplying an electric the mains and fordecentralized power supply, whereby wherein upon failure of one inverter(WR1-WR3) of the plurality of inverters, an asymmetrical power supplydistribution is reduced by limiting the output of the other inverters ofthe plurality of inverters, whereby wherein the failure of the oneinverter (WR1-WR3) is registered by a measurement signal of anadditional circuit of each of the inverter other inverters, acommunication signal resulting from the measurement signal beingprovided for limiting the output or for disconnecting the otherinverters from the mains, whereby wherein the measurement signal isgenerated by a current of a constant current source, said current ofsaid constant current source being in the a milliampere range.
 2. Themethod as set forth in claim 1, whereby that wherein the output of eachinverter (WR1-WR3) of the other inverters is durably limited to 2-20kVA, more specifically to about 4.6 kVA.
 3. The method as set forth inclaim 1, whereby wherein the output of each inverter (WR1-WR3) of theother inverters is limited temporarily, more specifically for about 10minutes, to about 5 kVA.
 4. The method as set forth in claim 1, wherebywherein the communication signal is communicated to the processors ofeach of the other inverters (WR1-WR3) for limiting the output of each ofthe other inverters by means of processor control.
 5. The method as setforth in claim 1, whereby the wherein a communication takes placebetween the plurality of inverters (WR1-WR3), through which eachinverter communicates through one or a plurality of external cables thatit is ready for supply.
 6. The method as set forth in claim 1, wherebythe wherein a communication takes place between the plurality ofinverters (WR1-WR3), each inverter communicating by radio that it isready for supply.
 7. A circuit arrangement comprising means forimplementing the method as set forth in claim 1, with a plurality ofsingle-phase inverters (WR1-WR3) of a decentralized power supply system,wherein each inverter has a failure measuring circuit (1) for reducingthe power of the other inverters inverter upon an asymmetrical powersupply resulting from the failure of one of the inverter inverters. 8.The circuit arrangement as set forth in claim 7, whereby wherein eachmeasuring circuit includes a direct current source (3), morespecifically a constant current source, that may be additionally butonly one constant current source is connected through an electricallyconductive bridge (4), so that only one of the current sources (3)delivers the in order to deliver a measurement current, each measuringcircuit comprising a connection for a measurement and/or output signalin such a manner that a failure or an OK signal can be communicated tothe other two inverters.
 9. The circuit arrangement as set forth inclaim 7, whereby wherein each inverter is implemented with an externalcommunication line (2).
 10. The circuit arrangement as set forth inclaim 7, whereby wherein a plurality of groups (GR1, GR2) of threesingle-phase inverters (11-16) is connected to the an output side of athree-phase protection switchgear, said switchgear being adapted to beswitched off through the a manual, external clearing means (18).
 11. Thecircuit arrangement as set forth in claim 10, whereby wherein theswitchgear is a contactor (19).
 12. The circuit arrangement as set forthin claim 7, whereby an implementation such that wherein, upon failure ofthe one inverter of the inverters, all the inverters are disconnectedfrom the mains when the voltage of one phase exceeds, or falls short of,imposed limit values.
 13. The circuit arrangement as set forth in claim12 whereby an implementation such that 10 wherein all the inverters aredisconnected from the mains by directly switching them off, a manual,external clearing means (18) being directly connected to all theinverter plurality of groups.
 14. A method comprising: applying ameasurement current in a milliampere range from a constant currentsource to an inverter system comprising a first single phase inverter, asecond single phase inverter and a third single phase inverter;monitoring the inverter system through a first circuit in the firstsingle phase inverter; monitoring the inverter system through a secondcircuit in the second single phase inverter; monitoring the invertersystem through a third circuit in the third single phase inverter; upona failure of the third single phase inverter, measuring the failure inthe first single phase inverter and the second single phase inverter;reducing a first output power of the first single phase inverter basedon the measured failure in the first single phase inverter; and reducinga second output power of the second single phase inverter based on themeasured failure in the second single phase inverter thereby reducing anasymmetrical power supply.
 15. The method according to claim 14, whereinmeasuring the failure comprises communicating a first communicationsignal to a first processor of the first single phase inverter andcommunicating a second communication signal to a second processor of thesecond single phase inverter.
 16. The method according to claim 14,wherein reducing the first output power and the second output powercomprises limiting the first output power and the second output power to2 kVA-20 kVA, respectively.
 17. A method of converting a direct voltageinto a three phase alternating voltage for supplying into a mains, eachsingle phase inverter of a plurality of single phase inverters having adifferent phase, the method comprising: monitoring a plurality of singlephase inverters by measuring a constant current in a milliampere rangeat a circuit of each single phase inverter; upon measuring a failure ofone of the single phase inverters, communicating the failure to aprocessor of each of the other single phase inverters; and reducing anasymmetrical power supply by limiting an output power of each of theother single phase inverters.
 18. The method according to claim 17,wherein reducing the output power comprises limiting the output power to2 kVA-20 kVA, respectively.
 19. A method comprising: monitoring anoperating condition of a first single phase inverter, a second singlephase inverter and a third single phase inverter by measuring a constantcurrent in a milliampere range; upon measuring a failure of the firstsingle phase inverter in circuits of the second single phase inverterand the third single phase inverter; and reducing an asymmetrical powersupply by reducing output powers of the second single phase inverter andthe third single phase inverter by communicating to processors of thesecond single phase inverter and the third single phase inverter thefailure of the first single phase inverter.
 20. The method according toclaim 19, wherein reducing the output power comprises limiting theoutput power to 2 kVA-20 kVA.
 21. A system comprising: a plurality ofsingle-phase inverters, each single phase inverter configured to providea different single phase and each single-phase inverter comprising amonitoring circuit configured to measure a constant current in amilliampere range; and a controller configured to control a power outputof the respective single phase inverter; and a connection connecting themonitoring circuits of the plurality of the single phase inverters,wherein upon a failure of one of the single phase inverters, themonitoring circuits of the other single phase inverters send acommunication signal to their respective controllers, and wherein thecontrollers of the other single-phase inverters reduce the power outputsof the other single-phase inverters to reduce an asymmetrical powersupply.
 22. The system according to claim 21, wherein the power outputsof the other single-phase inverters are limited to 2 kVA-20 kVA.
 23. Thesystem according to claim 21, further comprising a communication circuithaving one or a plurality of external cables.
 24. The system accordingto claim 21, further comprising a wireless communication circuit.
 25. Asystem comprising: a first single phase inverter configured to produce afirst single phase, the first single phase inverter comprising a firstmonitoring circuit; a first constant current source configured toprovide a current in a milliampere range; and a first controllerconfigured to control a first power output of the first single phaseinverter; a second single phase inverter configured to produce a secondsingle phase, the second single phase inverter comprising a secondmonitoring circuit; and a second controller configured to control asecond power output of the second single phase inverter; a third singlephase inverter configured to produce a third single phase, wherein thefirst, second and third single phase are different; and a connectionconnecting the first single-phase inverter, the second single-phaseinverter and the third single phase inverter, wherein the monitoringcircuit of the first single phase inverter is configured to send, upondetection of a failure of the third single phase inverter, a firstcommunication signal to the controller of the first single phaseinverter, wherein the monitoring circuit of the second single phaseinverter is configured to send, upon detection of the failure of thethird single phase inverter, a second communication signal to thecontroller of the second single phase inverter, wherein the first singlephase inverter reduces a first output power, wherein the second singlephase inverter reduces a second output power, and wherein anasymmetrical power supply is reduced.
 26. The system according to claim25, wherein the first output power comprises 2 kVA-20 kVA, and whereinthe second output power comprises 2 kVA-20 kVA.
 27. The system accordingto claim 25, wherein the first output power comprises about 4.6 kVA, andwherein the second output power comprises about 4.6 kVA.
 28. The systemaccording to claim 25, further comprising a communication circuitcomprises one or a plurality of external cables.
 29. The systemaccording to claim 25, further comprising a wireless communicationcircuit.
 30. The system according to claim 25, wherein the first singlephase inverter comprises a conductive bridge between the constantcurrent source and the monitoring circuit.